The present invention relates to an image sensor, and more particularly to an image sensor formed of a line sensor such as a CCD (Charge Coupled Device) sensor, suitable for reading an original in facsimile.
Recently, a line (image) sensor formed of an IC sensor, e.g., a CCD sensor consisting of a plurality of photodetecting elements (picture elements) such as photodiodes linearly arranged in a semiconductor substrate, has been used as an image sensor for reading originals in facsimile. A CCD sensor of this type is formed by applying, for example, a photoetching process to a silicon wafer, followed by cutting the wafer into pellets of a prescribed shape. The length of the sensor in the main scanning direction in the reading step is restricted by the wafer size. It follows that it is necessary to use a magnifying or reducing optical lens system in order to use a single CCD sensor (line sensor) for achieving a magnified or reduced reading of the ordinary original of A4 size, B4 size, etc.
In order to achieve a magnified or reduced reading of the original without using a magnifying or reducing optical lens system, it is necessary to use an image sensor device formed of a plurality of, e.g., 3 to 5, CCD sensors arranged such that the image sensor device has a length equal to that of the original. In this case, the CCD sensors are generally arranged linearly. Where the CCD sensors are linearly arranged, however, a remarkable problem takes place. Specifically, in accordance with an increase in the density of the picture elements, the irregularity in the cut plane at the edge of the CCD sensor is made noticeable relative to the size of the picture element, with the result that no picture element is provided in some of the junctions between the adjacent CCD sensors. In other words, the image sensing device tends to be incapable of reading a character or the like at some of the junctions between the adjacent CCD sensors. In fact, the pitch of the picture elements is 62.5 .mu.m, if the density of the picture elements is 16 pl (picture elements/mm.) It is unavoidable, however, for the slicing machine, generally used to provide about 10 .mu.m of the minimum irregularity of the cut plane of the CCD sensor. In addition, it is necessary to allow for 20-30 .mu.m of defective region, including the irregularity mentioned above. It follows that it is impossible to form a row of picture elements having a constant width of 58.5 .mu. m within the pitch of 62.5 .mu.m mentioned above. In other words, at least one picture element is unavoidably omitted in the junction between the adjacent CCD sensors.
To overcome the above-noted defect, the idea of arranging a plurality of sensors in a zig-zag fashion to form two rows of sensors is disclosed in, for example, FIG. 3 of Japanese Patent Disclosure (Kokai) No. 57-109664 and in Japanese Patent Disclosure (Kokai) No. 57-129065. In these cases, however, the CCD sensors included in one row differ from those included in the other row in the reading position of the original in the subscanning direction. In order to obtain output signals aligned in the subscanning direction, it is necessary to provide a line memory in the latter stage of the sensor portion. In addition, it is necessary to eliminate the overlapping in the junction portion of the CCD sensors in order to obtain signals serially aligned in the main scanning direction, leading to a complex control circuit. In conclusion, the system of arranging CCD sensors in a zig-zag fashion leads to a complex mechanism of the reading apparatus.
The omission of the picture element derived from the irregularity in the cut plane of the CCD sensor also remains unsolved in the case where a magnified or reduced reading of the original is performed with a magnifying or reducing optical lens system interposed between at least one CCD sensor and the original. Specifically, the photodetection element in at least one of the front and rear edges of the single CCD sensor, is adversely affected by the defective region, i.e., a picture element-nonforming region, including the irregularity of the cut plane, resulting in failure to form the picture element at said region.